1. Field of the Invention
The present invention relates to a method for refining domestic edible oils using non-aqueous membranes. More particularly, the present invention relates to a method for refining edible oils by membrane separation technology using a polyetherimide membrane.
During the past decade, considerable interest has been focused upon the processing of edible oils such as soybean oil, cotton seed oil, corn oil and the like. In the processing of such oils, it has been common to treat the oil with an organic solvent such as hexane to obtain miscella. Following, the organic solvent is removed, so yielding a crude glyceride oil composition. Unfortunately, the crude oils so obtained typically comprise up to 10%, by weight, of impurities including phospholipids, organic sulfur compounds, waxes, dye compounds and the like. These impurities typically have adverse affects and it is necessary to remove them from the crude oil to enhance the usefulness and marketability of the product.
Workers in the art discovered that these impurities could be effectively removed from the crude oil by means of different types of industrial membranes, selection of a particular membrane being dependent upon the materials to be separated. Thus, for example, microfiltration involves the membrane separation of macro molecules ranging from 500 to 2 million Angstrom units, ultra filtration with molecules ranging is size from 40-2,000 Angstroms and reverse osmosis with ions and molecules ranging in size up to 20 Angstroms.
In accordance with the ultrafiltration/reverse osmosis process, a crude glyceride oil composition is diluted with an organic solvent such as hexane and contacted with an ultrafiltration reverse osmosis membrane, typically selected from among polysulfones, polyacrylonitriles, and various polyamides which remove the solvent and yields a membrane permeable solution containing a degummed oil. Although, such techniques have met with a certain degree of success, inherent limitations have precluded the total exploitation thereof. More specifically, the complete removal of phospholipids from the crude oil is impeded by the characteristics of the membrane employed, so necessitating the use of a plurality of processing steps including, degumming, refining, bleaching and deodorizing to remove free fatty acids, phosphatides, particulates, chlorine materials and the like. Considerable amounts of energy in the form of steam or electricity are required in effecting these processes, each of which is required to obtain properly processed crude oil. Accordingly, workers in the art have focused their interest upon the development of alternate methods for refining the edible oils.
2. Description of the Prior Art
Typical prior art references describing methods for effecting this end are as follows:
U.S. Pat. No. 3,370,102 describes a process for separating a feed into a permeate stream and a retentate stream and utilizes a sweep liquid to remove the permeate from the face of the membrane to thereby maintain the concentration gradient driving force. The process can be used to separate a wide variety of mixtures including various petroleum fractions, naphthas, oils and hydrocarbon mixtures. Expressly recited is the separation of aromatics from kerosene.
U.S. Pat. No. 2,958,656 teaches the separation of hydrocarbons by type, i.e., aromatic, unsaturated, saturated, by passing a portion of a mixture thereof through a non-porous cellulose ether membrane and removing permeate from the permeate side of the membrane using a sweep gas or liquid. Feeds include hydrocarbon mixtures, naphtha (including virgin naphtha, naphtha from thermal or catalytic cracking, etc.).
U.S. Pat. No. 2,930,754 teaches a method for separating hydrocarbons e.g. aromatic and/or olefins from gasoline boiling range mixtures, by the selective permeation of the aromatic through certain cellulose ester non-porous membranes. The permeated hydrocarbons are continuously removed from the permeate zone using a sweep gas or liquid.
U.S. Pat. No. 4,115,465 teaches the use of polyurethane membranes to selectively separate aromatics from saturates via pervaporation.
U.S. Pat. No. 4,366,062 teaches reverse osmosis using a composite isocyanurate membrane. The method selectively separates at least one water soluble material from an aqueous solution. The membrane comprises a microporous substrate and a barrier layer about 0.01 to 0.1 micron thick. It is composed of a cross-linked polymeric material having an isocyanurate structure and substituents appended thereto selected from among hydrogen, glycidyl groups and alkyl radical groups having from 2 to 5 carbon atoms which may also contain functional hydroxyl groups or glycidyl groups. The crosslinked polymeric material has ester or ether linkages or combinations thereof connecting the isocyanurate structures to each other. There are no urethane groups present.
U.S. Pat. No. 4,557,949 teaches a method for making the reverse osmosis semipermeable membrane disclosed in U.S. Pat. No. 4,366,062.
European Application 0044872 teaches selectively separating water soluble materials from a solution under reverse osmosis conditions using a membrane having a porous support layer carrying a barrier layer of cross-linked isocyanurate polymer.
Japanese Application 81/160960 teaches an isocyanurate network terpolymer useful for the production of a selective permeation membrane. A polymer having hydroxyl groups and tertiary amine groups in the side chain is reacted with cyanuric chloride and subjected to terpolymerization by reacting the tertiary amine groups with the resultant hydrochloride to give a polymerized polyisocyanurate. A polymer made using glycidyl methacrylatestyrene copolymer, diethyl amine in benzene and methanol was produced having a 2-hydroxy-3-diethylaminopropyl group. This (polytetra fluorethylene) polymer was crosslinked with cyanuric chloride and cast on a PTFE plate and kept for 24 hours at 40.degree. to give a 44 .mu. membrane. This membrane was used to separate a mixture of cyclohexane and benzene under pervaporation conditions. A permeate gas which was 100% benzene was recovered at a rate 0.0025 g/m.sup.2 -hr.
U.S. Pat. 4,787,981 (Tanashi, et al.) which issued on Nov. 29, 1988, discloses a process for the purification of crude glyceride oil compositions which comprises diluting a crude glyceride oil containing gums and waxes with an organic solvent and contacting the diluted crude oil composition with a semipermeable membrane comprising a polyimide. The resultant semipermeable membrane permeable liquid is then bleached with a clay and deodorized to obtain a purified glyceride oil.
U.S. Pat. No. 4,929,357 (R.C. Schucker) discloses the use of a non-porous isocyanurate-crosslinked polyurethane membrane for separating aromatic from non-aromatic hydrocarbons.
U.S. Pat. No. 4,929,358 (B.A. Koenitzer) discloses the separation of aromatic hydrocarbons from non-aromatics by permeation of the aromatic compound through a polyurethane-imide membrane under pervaporation or reverse osmosis conditions.
Although each of the foregoing processes has advanced the art, the search has continued for the development of membrane separation technology of enhanced energy efficiency which enables workers in the art to economically isolate edible oils.